The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to ECG triggered vascular imaging using fast NMR pulse sequences for both the pulmonary vasculature and the peripheral vasculature where pulsatile blood flow is prevalent.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance ("NMR") phenomena is exploited.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as the polarizing field B.sub.0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. There is a class of pulse sequences which have a very short repetition time (TR) and result in complete scans which can be conducted in seconds rather than minutes. Whereas the more conventional pulse sequences have repetition times TR which are much greater than the spin-spin relaxation constant T.sub.2 so that the transverse magnetization has time to relax between the phase coherent excitation pulses in successive sequences, the fast pulse sequences have a repetition time TR which is less than T.sub.2 and which drives the transverse magnetization into a steady-state of equilibrium. Such techniques are referred to as steady-state free precession (SSFP) techniques and they are characterized by a cyclic pattern of transverse magnetization in which the resulting NMR signal refocuses at each RF excitation pulse to produce an echo signal. This echo signal includes a first part S+ that is produced after each RF excitation pulse and a second part S- which forms just prior to the RF excitation pulse.
There are two well known SSFP pulse sequences used to produce images. The first is called gradient refocused acquired steady-state (GRASS) and it utilizes a readout gradient G.sub.x to shift the peak in the S+ signal that is produced after each RF excitation pulse toward the center of the pulse sequence. In two-dimensional imaging, a slice selection gradient pulse is produced by the gradient G.sub.z and is immediately refocused in the well-known manner. A phase encoding gradient pulse G.sub.y is produced shortly thereafter to position encode the acquired NMR data, and to preserve the steady-state equilibrium, the effects of the phase encoding gradient pulse are nullified by a corresponding G.sub.y rewinder gradient pulse after the NMR signal has been acquired and before the next pulse sequence begins as described in U.S. Pat. No. 4,665,365.
The second well known SSFP pulse sequence is called contrast enhanced fast imaging (SSFP-ECHO) and it utilizes the S- signal that is produced just prior to each RF excitation pulse. In this pulse sequence the acquired NMR signal is an S- echo signal caused by the gradient refocusing of the transverse magnetization which would otherwise refocus at the next RF excitation pulse. The readout gradient G.sub.x is substantially different in this pulse sequence and includes a positive pulse prior to the actual readout pulse and a negative pulse after the readout pulse. The former pulse dephases the FID signal (S+) which might otherwise be produced during the data acquisition window, and the latter pulse causes the transverse magnetization to rephase during the next pulse sequence to produce the echo signal S-. For a more detailed discussion of the SSFP-ECHO pulse sequence, reference is made to an article by R. C. Hawkes and S. Patz entitled "Rapid Fourier Imaging Using Steady-State Free Precision", published in Magnetic Resonance in Medicine 4, pp. 9-23 (1987).
The fast NMR pulse sequences can be used to great advantage when imaging the vasculature of the lungs. Since a complete slice using such a pulse sequence can be acquired in approximately one second, it is possible in a single breath hold by the subject to acquire a series of 2D slices. This is in contrast to conventional techniques which require minutes for each slice and must employ respiratory gating to reduce blurring caused by respiratory movements.
To enhance the contrast between pulmonary vessels and surrounding tissues, the acquisition of the NMR data is synchronized with the subject's cardiac cycle. For example, maximum signal intensity in the pulmonary arteries is attained in the late systole or early diastole portions of the cardiac cycle. Since image contrast is determined primarily by the central, or low spatial views of the scan, this suggests that the start of each scan be delayed for a specific time interval after the detection of the cardiac trigger such that the central views are acquired at the proper moment.
Such delays of 50 to 600 milliseconds have little impact on the scan time using conventional pulse sequence, but the impact on fast pulse sequence pulmonary vascular imaging can be substantial. This is illustrated in FIG. 1, which depicts an ECG signal 10 produced at a heart rate of 60 beats/minute. The R-R interval of the ECG signal 10 is approximately one second, and using fast sequences a complete slice acquisition 11 with 128 views can be acquired during each R-R interval if it is commenced at the beginning of the cardiac cycle when the ECG trigger signal is generated. Consequently, a typical scan of 12 to 16 slices can be acquired within 16 seconds, which can be accomplished within a single breath hold of even an infirm patient. However, if the slice acquisition is delayed for 400 milliseconds in order to attain higher signal intensity in the arteries, the slice acquisition overlaps into the next cardiac cycle as shown at 12. As a result, two one second cardiac cycles are required for each slice acquisition 12 and the total scan time is doubled. The resulting 24 to 32 second scan is very difficult for some patients to complete in a single breath hold, and the primary advantage of using the fast pulse sequence in this application is lost.
Although the method of a variable ECG delay by view reordering is particularly applicable in imaging the pulmonary vessels, the method can also be applied to vascular imaging in any region where pulsatile flow is a problem. Pulsatile flow generates artifacts in conventional acquisition pulse sequences which degrades image quality and prevents accurate diagnosis. Since the sequence repetition times (TR) for a conventional vascular imaging sequence is of the order of 30-50 ms, conventional cardiac gating where a single line of k-space is acquired per cardiac trigger results in image acquisition times of the order of 2 min per image. In order to generate a time-of-flight vascular imaging, several sections are required. This poses a problem in peripheral vascular imaging where a 30-60 cm region must be covered. Conventional ECG gating is impractical as the scan time is about 60 min for a complete series of images. Variable ECG delay by view reordering permits an image to be acquired in 1-2 seconds. Thus, a series of 30-60 images necessary to generate a time-of-flight vascular image requires a scan time of 1-2 minutes. This represents a significant time savings in addition to improving the image quality.